// 绝对定位 参数：上, 右, 下, 左
@mixin position-absolute($top: null, $right: null, $bottom: null, $left: null) {
    position: absolute;
    
    @if $left != null and $left != "" {
      left: $left;
    }
    @if $right != null and $right != "" {
      right: $right;
    }
    @if $top != null and $top != "" {
      top: $top;
    }
    @if $bottom != null and $bottom != "" {
      bottom: $bottom;
    }
  }
  